As is well known, electronic devices generate electromagnetic emissions. These emissions are undesirable for a number of reasons, in particular since they may interfere with other electronic devices, such as telephones, televisions and radios. In consequence, it is conventional to use a metallic casing to cover and shield many types of electronic apparatus.
Personal computers, being electronic devices, are no exception to this rule. Personal computers generate unwanted electromagnetic emissions and the faster the processor, the worse is the interference problem. Standards have been established by the Federal Communications Commissions (FCC) and other standard setting bodies which strictly limit the amount of EMI emitted from electronic devices. Meeting these standards in a cost-effective way is a significant technical challenge.
Whilst the use of a metallic casing can be effective in reducing unwanted emission of electromagnetic radiation, at high frequencies further measures are necessary to keep EMI emissions down to the required levels. Modern personal computers use clock speeds of 200 MHz or more. Digital circuits operating at this speed can cause electromagnetic radiation to be emitted at frequencies in excess of 1 GHz. At such high frequencies, the wavelength of the emitted radiation can be such that the joints between different pieces of a casing can be a weak point where small gaps can allow electromagnetic radiation to leak out.
To alleviate this problem, an EMI gasket or sealing strip can be used in order to provide a tight spatial contact and an electromagnetic connection between two mating pieces of a casing. Examples of this kind of gasket can be found in U.S. Pat. No. 4,659,869 and U.S. Pat. No. 5,015,802.
Many different types of such gaskets are commercially available for different applications. Whilst these are generally satisfactory, they do suffer from a number of drawbacks.
Typically, the gaskets that are used in personal computers are made of resilient beryllium copper and, commonly need to be fixed in place using some kind of adhesive. This material is relatively expensive and the very thin sheets of the material which are used can have sharp edges. These sharp edges can lead to safety problems both in manufacturing and in the use of the device concerned. The need to use an adhesive to fix the gasket complicates the manufacturing process and can lead to difficulties in disposing of or recycling the apparatus at the end of its useful life. Moreover, if the gasket is made from a different metal than the casing itself, problems can arise due to chemical reaction between the two different metals in the gasket and the casing.
Another problem with conventional gaskets is that it is often difficult to avoid stressing the resilient material of the gasket beyond its elastic limit. This can lead to an irreversible deformation or crushing of the gasket which reduces the effectiveness of the shielding once the casing has been opened and closed a few times. This latter problem is particularly acute in the case of personal computer casings, since these are usually designed so that option cards of various types can be mounted and removed by a user to the interior of the casing. In contrast to other types of electronic apparatus, such as video or audio equipment, a personal computer casing may need to be opened and closed many times during its life.
The object of this invention is to provide a simple reduced-cost casing gasket design which mitigates the above drawbacks.